Brodies Firm GDPR 2018 [pg. 2] explains that the sample it surveyed resembles U.K. businesses as a whole by industry and by revenue or number of employees.
Osborne Clarke Consumer 2018 [pg. 28] strived to balance its participants within each nation.
White Case Arbitration 2010 [pg. 40] excellently describes its efforts to reach out and obtain a representative group of participants.
It’s very unusual for reports in a series to point out differences in the participant pools. Here is one example, however. Baker McKenzie Cloud 2016 [pg. 5] acknowledges that the 2016 survey has more respondents who are lawyers (“in a legal role”) than previous surveys.